U.S. patent number 9,664,636 [Application Number 14/138,933] was granted by the patent office on 2017-05-30 for chlorine detection with pulsed amperometric detection.
This patent grant is currently assigned to Thermo Fisher Scientific Aquasensors LLC. The grantee listed for this patent is Thermo Fisher Scientific Aquasensors LLC. Invention is credited to Arthur E. Tobey, Xiaowen Wen.
United States Patent |
9,664,636 |
Wen , et al. |
May 30, 2017 |
**Please see images for:
( Certificate of Correction ) ** |
Chlorine detection with pulsed amperometric detection
Abstract
A method of measuring oxidant in an aqueous composition using an
electrode system comprising a membrane permeable to a species to be
measured, a cathode situated behind the membrane, and an
electrolyte between the membrane and cathode. Embodiments of the
method may include contacting the membrane and an anode with the
aqueous composition, and measuring the current between the cathode
and anode in response to a first voltage applied between them. A
pulse of a second voltage may be applied between the cathode and
the anode, where the second voltage is different from the first
voltage. Apparatus and computer program products which may perform
the method, are also provided.
Inventors: |
Wen; Xiaowen (Lexington,
MA), Tobey; Arthur E. (Salem, NH) |
Applicant: |
Name |
City |
State |
Country |
Type |
Thermo Fisher Scientific Aquasensors LLC |
Waltham |
MA |
US |
|
|
Assignee: |
Thermo Fisher Scientific
Aquasensors LLC (Waltham, MA)
|
Family
ID: |
53275378 |
Appl.
No.: |
14/138,933 |
Filed: |
December 23, 2013 |
Prior Publication Data
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|
Document
Identifier |
Publication Date |
|
US 20150177173 A1 |
Jun 25, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N
27/4045 (20130101); G01N 27/26 (20130101) |
Current International
Class: |
G01N
27/40 (20060101); G01N 27/26 (20060101); G01N
27/404 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2778463 |
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Nov 1999 |
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FR |
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S62153740 |
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Jul 1987 |
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JP |
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2013014187 |
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Jan 2013 |
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WO |
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Other References
Austin-Harrison et al., "Pulsed Amperometric Detection Based on
Direct and Indirect Anodic Reactions: A Review," Electroanalysis,
1, 189-197, 1989. cited by applicant .
Coutinho et al., "Development of Instrumentation for Amperometric
and Coulometric Detection using Ultramicroelectrodes," J. Braz.
Chem. Soc., 19(1), 131-139, 2008. cited by applicant .
Damlin et al., "Characterization of Hardwood-derived
Carboxymethylcellulose by High pH Anion Chromatography Using Pulsed
Amperometric Detection," Cellulose Chem. Technol., 44(1-3), 65-69,
2010. cited by applicant .
Handbook of Ion Chromatography, 3rd completely revised and updated
edition, 2004 WILEY-VCH Verlag GmbH & Co., KGaA, Weinheim,
ISBN: 3-527-28701-9 (see section 7.1.2 "Amperometric Detection").
cited by applicant .
Huntley and Malkov, "Amperometric Probes or DPD Analyzers: Which is
Best for On-Line Chlorine Monitoring?," WaterWorld, Editorial
Feature, 2 pages, 2009. cited by applicant .
Johnson et al., "Chromatography with Pulsed Electrochemical
Detection at Gold and Platinum Electrodes," Anal. Chem., 62(10),
589 A-596 A, 1990. cited by applicant .
Lacourse, "Pulsed Electrochemical Detection: Waveform Evolution,"
Chromatography Online.com, 11 pages, Jul. 1, 2011. cited by
applicant .
Malkov et al., "Comparison of On-line Chlorine Analysis Methods and
Instrumentation Built on Amperometric and Coloimetric
Technologies," American Water Works Assocation, 22 pages, copyright
2009. cited by applicant .
Neuburger et al., "Pulsed Amperometric Detection of Carbohydrates
at Gold Electrodes with a Two-Step Potential Waveform," Anal.
Chem., 59, 150-154, 1987. cited by applicant .
Waters 2465 Electrochemical Detector, Operator's Guide,
71500246502, Rev. B, 226 pages, copyright 2002, 2007. cited by
applicant .
Great Britain Intellectual Property Office, Corresponding GB Patent
Application No. 14229652, Search Report Under Section 17, Sep. 28,
2015, 2 pages. cited by applicant.
|
Primary Examiner: Ball; J. Christopher
Attorney, Agent or Firm: Wood Herron & Evans LLP
Claims
The invention claimed is:
1. A method of measuring oxidant in an aqueous composition using an
electrode system comprising a membrane permeable to a species to be
measured, a cathode situated behind the membrane, and an
electrolyte between the membrane and cathode, the method
comprising: contacting the membrane and an anode with the aqueous
composition and measuring the current between the cathode and anode
in response to a first voltage applied between them; applying a
pulse of a second voltage between the cathode and the anode, where
the second voltage is different from the first voltage.
2. A method according to claim 1 wherein the cathode is comprised
of gold or platinum.
3. A method according to claim 1 wherein the pulse is applied when
the electrode system is in contact with a chlorine free aqueous
composition.
4. A method according to claim 3 wherein the electrode system is in
contact with the chlorine free aqueous composition for at least 1
hour and the pulse of the second voltage is applied during that
time.
5. A method according to claim 3 wherein the pulse of the second
voltage is applied repeatedly during a time in which the electrode
system is in contact with the chlorine free aqueous
composition.
6. A method according to claim 3 wherein the electrode system is in
contact with the chlorine free aqueous composition for at least 5
hours and the pulse of the second voltage is applied during that
time.
7. A method according to claim 1 additionally comprising
sequentially and repeatedly exposing the membrane to a chlorine
containing aqueous composition and a chlorine free aqueous
composition, and wherein multiple pulses of the second voltage are
applied during each of multiple periods during which the membrane
is exposed to the chlorine free aqueous composition.
8. A method according to claim 4 additionally comprising applying
multiple pulses of the second voltage during each of multiple
periods of exposure to the chlorine containing composition.
9. A method according to claim 4 wherein the second voltage
comprises a voltage which is initially more negative than the first
voltage.
10. A method according to claim 4 wherein the total chlorine
concentration of the chlorine containing composition is less than
20 ppm.
11. A method according to claim 4 wherein the pulses of the second
voltage comprises a voltage which differs from the first voltage by
less than 200 mV.
12. A method according to claim 4 wherein the voltage pulses are
applied at a frequency of no more than 10 Hz.
13. A method according to claim 1 wherein a T90 in response to the
first voltage is less than 70% of the T90 measured under the same
conditions but without applying the pulse of the second voltage,
the T90 being defined as the time needed to reach 90% of a steady
state signal after a change in a concentration from 0 ppm to 1.1
ppm of the oxidant.
14. A computer program product carrying a computer program in a
non-transitory form which program, when loaded into a processor,
executes a method of measuring free chlorine in an aqueous
composition comprising: applying a first voltage between an anode
and a cathode having an aqueous composition therebetween; measuring
the current between the anode and cathode in response to the first
voltage applied between them; calculating free chlorine
concentration of the aqueous composition based on the measured
current; applying a pulse of a second voltage between the anode and
cathode, wherein the second voltage comprises a voltage which is
initially more negative than the first voltage.
15. A computer program product according to claim 14 wherein a
sequence of alternating first and second voltages are applied at a
frequency of no more than 10 Hz.
16. A computer program product according to claim 14 wherein the
calculating free chlorine comprises calculating total free
chlorine.
Description
FIELD
This invention generally relates to detecting a component by
amperometric detection.
BACKGROUND
Detecting the presence and concentration of different chemical
species using amperometry is a very well known technique.
Basically, a voltage appropriate for the species to be detected is
applied between two electrodes in communication with a fluid
possibly containing that species. The voltage is selected which is
sufficient to cause oxidation or reduction of the species to be
detected but ideally not any other species in the fluid. The
resulting current between the electrodes in response to the applied
voltage can then provide an indication of the presence or
concentration of the species to be detected. In coulometry the
total current over a period of time is measured. In the present
application "amperometry" will be used to include methods such as
coulometry or voltammetry. Descriptions of amperometry and methods,
including selection of the appropriate voltage, are widely
available in references including the following and references
cited therein: (i) Handbook of Ion Chromatography, Third Completely
Revised Edition, 2004 WILEY-VCH Verlag GmbH & Co. KGaA,
Weinheim, ISBN: 3-527-28701-9 (see section 7.1.2 "Amperometric
Detection"); (ii) "Pulsed Amperometric Detection Based on Direct
and Indirect Anodic Reactions: A Review", Electroanalysis, 1 (1989)
189-197; (iii) "Development of Instrumentation for Amperometric and
Coulometric Detection using Ultramicroelectrodes", J. Braz. Chem.
Soc., Vol. 19, No. 1, 131-139, 2008.
Amperometry has been used to detect a large variety of organic
species. Additionally, amperometry has also been used to detect
chlorine in aqueous compositions, such as described in "Comparison
of On-line Chlorine Analysis Methods and Instrumentation Built on
Amperometric and Colorimetric Technologies", 2009, available from
the American Water Works Association, and French patent
FR2778463.
In cases involving the oxidation of aliphatic organic compounds at
an anode, where the electrolyte is in direct contact with the anode
and cathode, it has been recognized that the anode can become
fouled--see for example "Liquid Chromatography with Pulsed
Electrochemical Detection at Gold and Platinum Electrodes",
Analytical Chemistry, Vol. 62, No. 10, May 15.1990. In that
situation a technique known as pulsed amperometric detection has
been used in which a reading potential is applied, followed by a
large positive potential to oxidatively desorb adsorbed
hydrocarbons, then followed by a large negative potential to
cathodically dissolve oxides.
SUMMARY
The present invention recognizes that amperometric detection can be
used to detect oxidants (such as chlorine) by reduction at a
cathode positioned behind a membrane. The membrane may be
selectively permeable to one or more detectable components, for
example, an oxidant. Such oxidants can include chlorine in the form
of hypochlorous acid (HOCl) or hypochlorite anion (ClO.sup.-) or in
the combined form of chloramines. Other oxidants can include other
halogens such as bromine in the form of hypobromous acid,
hypobromite, or bromoamines. Some embodiments of the present
invention further recognize that even in this situation, where
reduction rather than oxidation is used to detect a species such as
free chlorine, and the cathode is behind a membrane, performance of
the system may still be adversely affected. Furthermore, some
embodiments of the present invention recognize that performance may
even be adversely affected where the electrode system is exposed to
a chlorine-free composition before being exposed to a chlorine
containing composition.
The present invention provides, in one embodiment, a method of
measuring a component, such as an oxidant (for example, chlorine)
in an aqueous composition. Such embodiments may use an electrode
system and membrane. The membrane is permeable to the species to be
measured, such as chlorine in the form of hypochlorous acid (HOCl)
or hypochlorite anion (ClO.sup.-) or in the combined form of
chloramines. A cathode is situated behind the membrane, and an
electrolyte is disposed between the membrane and cathode. The
electrolyte may contain components that facilitate chemical
reactions and equilibriums for the electrochemical detection. This
embodiment of the method may include contacting the membrane and an
anode with the aqueous composition and measuring the current
between the cathode and anode in response to a first voltage
applied between them. Additionally, a pulse of a second voltage is
applied between the cathode and the anode, where the second voltage
is different from the first voltage.
Another embodiment provides an apparatus for measuring a component
as described above, such as chlorine in an aqueous solution. Such
an apparatus may include an electrode system comprising an
electrolyte chamber defined at least in part by a membrane of a
type described above, and a cathode situated behind the membrane. A
power supply applies a first voltage and a pulse of a second
voltage between the cathode and an anode, where the second voltage
comprises a voltage which is different from the first voltage. A
current detector measures the current between the cathode and the
anode in response to the first voltage. As an example, the second
voltage may initially be more negative or positive than the first
voltage although it may also comprise a voltage which is more
positive than the first voltage (or more negative than the first
voltage).
Other embodiments include a computer program product carrying a
computer program in a non-transitory form. The computer program,
when loaded into a processor, may execute a method of the present
invention and may be used to control an apparatus of the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described with reference
to the drawings in which:
FIG. 1 illustrates an apparatus of an embodiment of the present
invention.
FIG. 2 is an enlarged view of part of FIG. 1 showing in more detail
a part of the electrode system.
FIG. 3 is a graph illustrating performance of an electrode system
under different conditions both with and without use of a method of
the present invention.
FIGS. 4-6 are similar to FIG. 3 but illustrate performance with and
without use of a method of the present invention under various
different conditions.
FIG. 7 is a flowchart illustrating a method of the present
invention.
FIGS. 8 and 9 illustrate waveforms that may be used in a method of
the present invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
In an embodiment of the present invention, the cathode may be gold,
platinum, or another noble metal, although other materials, such as
carbon and doped diamond, may be used. An anode may be made of any
suitable material, such as a material to act as both the reference
and counter-electrodes in a two-electrode system, for example
silver. However, other materials can be used for the anode or the
reference and counter electrodes in a three electrode system or in
systems with other numbers of electrodes. The membrane material may
be hydrophobic or hydrophilic type. The electrolyte may contain
components that facilitate chemical reactions and equilibriums. For
example, it can contain be buffered solution containing iodide
salts. The electrolyte may also contain polymers to control the
viscosity and to control the leaching of active components. The
referenced pulse may be applied when the electrode system is in
contact with a composition free of detectable species (e.g., a
chlorine-free composition) or with a composition containing the
detectable species (e.g., a chlorine-containing composition). In
some cases, the referenced pulses may be applied repeatedly for a
period of time before or between the actual measurements when the
electrode system is in contact with a chlorine-free or
chlorine-containing composition. For example, the electrode system
may be in contact with a chlorine-free or chlorine-containing
aqueous composition for a period of at least any one of 10 minutes,
30 minutes, 1 hour, 10 hours or 24 hours, and the pulse of the
second voltage may be applied at any time during that period. In
another option, the pulse of the second voltage may be combined
with the first voltage to form pulse cycle and the cycle may be
applied repeatedly and continuously when the electrode system is in
contact with the chlorine-free aqueous composition or the
chlorine-containing composition. As previously mentioned, the
detectable species may be an oxidant such as any halogen, for
example, chlorine, iodine or bromine. For example, in seawater, due
to the presence of bromide, all chlorine is converted to bromine as
a result of chlorine oxidizing bromide. In the detection of bromine
the reactions and methods are essentially the same as described
herein with hypobromous acid, hypobromite, and bromoamines
replacing the corresponding chlorine compounds wherever discussed.
This would also be the case for the detection of iodine.
The pulse of the second voltage, or the combined pulse cycle of the
first and second voltages may be applied at any time during the
period in which the electrode system is in contact with a
chlorine-free or chlorine-containing composition. They may also be
applied as a single pulse or a series of one or more pulses. In one
embodiment the pulses of the second voltages are applied after the
electrode system is in contact with the chlorine-free composition
for a period of time and prior to collecting measurements for
chlorine in the chlorine-containing composition. For example, the
pulses of the second voltages may be applied after the electrode
system is in contact with the chlorine-free composition for 0.5 to
24 hours but within any of 1 minute, 5 minutes, 30 minutes, 1 hour,
or 5 hours before the electrode system is then contacted with a
chlorine-containing composition and the first voltage is
applied.
It will be understood that when any reference is made in this
application to the pulse of the second voltage being applied at
certain times, either as a single pulse or a series of pulses, that
pulse may be applied only during any of the mentioned times or may
also be applied at additional times. Further, the first voltage may
also be applied only during the times referenced, or also applied
at additional times than when a current is to be measured. For
example, the second voltage and the pulse of the second voltage may
be applied in an alternating manner as a single waveform, with
optionally one or more other voltages applied between them. The
single waveform may take on many shapes, and may typically be a
rectangular waveform of fixed frequency such as no more than any
one of the frequencies between 100 Hz and 3 Hz, for example no more
than 5, 10 or 20 Hz.
In some embodiments the membrane of the electrode system is exposed
to a chlorine-containing aqueous composition for one period of time
and then to a chlorine-free aqueous composition for another period
of time, and this sequence repeated. For example, this may occur
where the method is used to regularly monitor variations in
chlorine at a fixed location in an industrial process. In any such
situation where the electrode system is sequentially and repeatedly
exposed to chlorine-free and chlorine-containing compositions,
multiple pulses of the second voltage may be applied during any one
or more or all of the multiple periods during which the membrane is
exposed to the chlorine-free aqueous composition. For example, this
may be accomplished by either applying one or more pulses of the
second voltage during exposure to only the chlorine-free
composition, or by applying a waveform of a type already described
during at least part or all of each of periods of chlorine-free and
chlorine-containing exposure.
The second voltage is different from the first voltage. It may be
composed of a single voltage waveform or of multiple voltage wave
forms with multiple steps of voltages and varied duration of each
step. For example, in one embodiment of the method, the second
voltage is composed of a square voltage step (E2) more negative
than the first voltage for duration of T2 followed by another
square voltage step (E3) more positive than the first voltage for
duration of T3. The second voltage can be combined with the first
voltage (E1) of duration T1 to form a full cycle of pulses, as
illustrated in FIG. 8. This full cycle of pulses may be applied
repeatedly, as illustrated in FIG. 9, at a regular frequency, such
as no more than any one of the frequencies between 20 Hz and 0.5
Hz. The second voltage (E2, E3) may deviate significantly from the
first voltage. The large excursions are intended to accomplish
surface cleaning and reactivation. In embodiments of this invention
the idea of PAD is used in a new application where an oxidant is
detected at a cathode surface that is indirectly in contact with
the sample by a membrane. Embodiments of this invention also
propose the use of minor voltage excursions, e.g., .+-.50-.+-.200
mV away from the first voltage. The minor voltage excursion helps
maintain the surface activity especially when analytical
measurement signal is near zero while not requiring long recovery
time after returning to measurement step at E1. Therefore,
measurement may be more continuous.
A wide range of chlorine concentrations may be measured. For
example, some embodiments may be used where the chlorine-containing
composition is less than any one of 50 ppm, 20 ppm, 10 ppm, 5 ppm,
or even less than 1 ppm. Generally, the chlorine-free and
chlorine-containing compositions may be aqueous compositions.
Further, in any embodiment parameters may be selected such to
shorten the response time T90 which is defined as the time needed
to reach 90% of the steady state signal after a change in the
concentration from 0 to 1.1 ppm of analyte being measured, such as
free chlorine. For example, T90 could be improved from 20 minutes
without applying the pulse of the second voltage to less than 5
minutes. In other examples, T90 may be less than 90%, less than 80%
or even less than 70% or 60% of the T90 measured under the same
conditions but without applying the pulse of the second voltage.
For example, the parameters selected to obtain this condition may
include the second voltage, the frequency of a waveform which
includes a pulse of the second voltage, the total time during which
such a waveform is applied, and the elapsed time after application
of that waveform and before the electrode is next exposed to a
chlorine-containing composition. For example, if T90 was 5 minutes
without applying the pulse of the second voltage, then a T90 of
less than 70% of that value would be less than 3.5 minutes.
While many embodiments of the invention herein will be described
with a cathode and an anode, it will be appreciated that other
embodiments may include more than two electrodes. For example, it
is well known in that a three electrode system can be configured
for amperometric measurements. Such a three electrode system has a
working electrode, e.g., cathode, a reference electrode and an
auxiliary electrode, e.g., anode, (the latter sometimes is
referenced as a counter electrode). Current measured in response to
the first voltage is total current through the working electrode,
regardless of the number of electrodes.
By a "chlorine-containing" composition is referenced a composition
which contains at least 0.5 ppm of chlorine, although they could
contain at least 1 ppm, 2 ppm, 3 ppm, 5 ppm or 10 ppm of chlorine.
"Free chlorine" references the concentration of hypochlorous acid
(HOCl). "Total free chlorine" references the concentration of HOCl
and hypochlorite anion (OCl.sup.-). "Total chlorine" references the
sum of the total free chlorine plus the concentration of the
chlorine combined with ammonia in the forms of chloramines. With a
hydrophobic (gas-permeable) membrane, only the HOCl is detected. To
determine total free chlorine based on a measurement of HOCl a pH
compensation factor is applied to convert the measured HOCl value
to value representing the total of HOCl and OCl.sup.-. However,
with hydrophilic membrane "total chlorine" can be detected. A
"chlorine-free" composition is one which is not chlorine-containing
(that is zero-chlorine), but may even contain less than 1 ppm or
less than 0.5, 0.2 ppm, or 0.1 ppm of free chlorine. In this regard
it is well known that molecular chlorine in an aqueous solution
will coexist with the other two forms of free chlorine in
equilibrium according to the equations:
Cl.sub.2+H.sub.2O=HOCl+H.sup.++Cl.sup.- Eq. (1); pK1=4.6 at
25.degree. C. HOCl=H.sup.++OCl.sup.- Eq. (2); pK2=7.5 at 25.degree.
C. As can be seen from the above, the coexistence of the different
forms is pH and temperature dependent. In the situation where the
membrane is permeable to only one of the forms, for example HOCl,
then the current measured in response to the first voltage
represents only the concentration of that form and free chlorine
concentration must therefore be calculated back from that
measurement in a known manner. pH measurement may be taken such as
by a pH sensitive electrode, and a pH compensation can be made in
free chlorine calculation in a known manner taking into account
Eqs. (1) and (2). Similarly, temperature may be measured and the
calculation can be adjusted for the measured temperature.
In any embodiment, an electrode or other item may "comprise" an
identified material, such as gold or platinum, or the electrode or
other item "is" a particular material such as gold electrode. By
"is" in this context allows the material to contain for up to 2%,
5%, 10%, or even 20% or 25% by weight of other materials, such as
other metals.
"Measuring", "identifying", "detecting" or similar terms, includes
either or both a qualitative evaluation (for example, the substance
is or is not present) as well as a quantitative evaluate (that is,
how much is present). "May" in this application references
something that is optional, for example if an item "may" be present
then that means that in one embodiment the item either is present
and in another embodiment it is not present. "Or" in this
application references any one, or all. For example, if "A or B" is
present, then this includes: only A is present; only B is present;
both A and B are present. "Contact" or "contacted" or similar
terms, are used interchangeably with "expose" or "exposed" or
similar terms. All voltages given, unless otherwise indicated, are
with reference to the cathode which is held at system ground
potential.
A "processor" as used herein may be any hardware or
hardware/software combination which is capable of carrying out the
steps require of it. For example, a processor may be a suitably
programmed microprocessor or application specific integrated
circuit. A processor may also include, or be used in conjunction
with, a memory of any known type such as a read-only or read-write
memory, which holds instructions and data for the operations as
described herein. A computer program product referenced herein may
include any suitable means for carrying the computer program in a
non-transitory form, such as a memory of any configuration, for
example a solid state memory or an optical or magnetic memory. The
operations or sequences of any method described in the present
application can be performed in the order described or in any other
order that is logically possible (including, where logically
possible, one or more operations being performed simultaneously).
All other references cited in this application are incorporated
into this application by reference, except to the extent to which
they may conflict with the present application in which event the
present application prevails. It will be understood throughout this
application that some or all of any of the different features
described can be combined with each other in different
combinations. For example, different ones of the features described
under this "Detailed Description" heading, may be used individually
or in any combination with the features described under the
"Summary" heading.
Referring now to FIGS. 1 and 2, the apparatus shown includes an
electrode system 10 with a generally cylindrical outer body 18
having a vent hole 22 on one side. A lower end 6 of outer body 18
is closed by a membrane 26. A solid inner body 30 encloses a gold
cathode 34 such that only a lower face 36 of cathode 34 is exposed
at a position behind membrane 26 while an upper end 38 is available
for an external electrical connection. A cylindrical silver
electrode 14 is wrapped around inner body 30, and has an upper end
16 available for an electrical connection. Silver electrode 14 will
be the anode and acts as both the reference and counter electrode
in the two electrode system shown. As mentioned before though,
separate reference and counter electrodes can be used in a three
electrode system. Note that there is a gap 40 between inner body 30
and outer body 18, as well as an almost invisible gap 44 between
face 36 and membrane 26. Gap 40 is filled with a reference
electrolyte to cover the whole electrode 14. Suitable reference
electrolytes include halide salt (e.g., KCl) solution at a 0.1 M
concentration. Gap 44 may or may not be visible but exists to allow
electric continuity between working electrode and the reference and
counter electrode, especially when reference electrode and counter
electrode are behind the membrane. Outer and inner bodies 18, 30
will be made of an inert material, such as a plastic or glass.
Membrane 26 may be a semi-permeable membrane with high permeability
to one or more of molecular chlorine (Cl.sub.2), hypochlorous acid
(HOCl), or hypochlorite anion (OCl.sup.-), and may have low
permeability to other species. It can also functions as a filter to
discriminate certain particles. Examples of suitable materials for
membrane 26 include hydrophobic type, such as
polytetrafluoroethylene ("PTFE"), polyvinylidene fluoride ("PVDF"),
and hydrophilic type, such as polyethersulfone (PES), polycarbonate
(PCTE), Polyvinylidene fluoride (PVDF). Since membrane 26 is
delicate, a mesh (not shown) of suitable material may be provided
for support.
Electrode system 10 is connected to a power supply 50 and current
detector 54, which includes analog-to-digital converter 58, and op
amplifier 56 connected between cathode 38 and ground. Power supply
50 applies the first voltage and the pulse of the second voltage
across cathode 34 and anode 14, while current detector 54 measures
the cathode current. A processor 60 controls power supply 50 and
converts current readings from current detector 54 into total free
chlorine readings for presentation on display 80 and/or storage in
memory 70. This operation may be in accordance with computer
program code stored in memory 70 and in response to operator input
received at input device 90. Power supply 50 may provide the first
voltage and a pulse of the second voltage in accordance with any of
the various programs described below, or any other suitable
program.
The apparatus of FIGS. 1 and 2 may be used in an embodiment of a
method of the present invention. One method may be applied
according to FIG. 7 in which the membrane 36 of the electrode
system is contacted (400) with a chlorine-free aqueous solution.
One or more pulses of a second voltage may be applied (410) during
this period of contact. Membrane 36 is then contacted (420) with a
chlorine-containing aqueous solution during which period a first
voltage may be applied (430) and the resulting current measured
(430). Processor 60 may calculate total free chlorine using the
measured current and any pH and temperature readings from sensors
(not shown). The membrane of the electrode system may again be
contacted (400) with a zero-chlorine aqueous composition and the
cycle (400-440) then repeated one or more times with a same or
different chlorine-containing solution and same or different first
and second voltages and their application parameters.
While not being intended to limit the present invention to any
specific electrochemical mechanism, it is believed that in a
2-electrode system the processes occurring during step 430 may be
as follows. Namely, while the first voltage is applied and the
current is being measured (430), chlorine species (e.g.,
hypochlorous acid HOCl) diffuses through membrane 26 and is reduced
at the cathode according to Eq. (3) below. A diffusion limited
equilibrium concentration between face 36 of gold cathode 34 and
membrane 26 is reached. HOCl+H.sup.++2e.sup.-=Cl.sup.-+H2O Eq. (3)
At the silver electrode (anode), silver is oxidized into Ag.sup.+
ions which may then precipitate with the halide ions (e.g.,
Cl.sup.-) present in the electrolyte according to Eq. (4) below:
2Cl.sup.-+2Ag=2AgCl+2e.sup.- Eq. (4)
The HOCl reduction at the cathode generates a current which is
directly proportional to its concentration when diffusion is under
control.
In a case that the membrane is hydrophilic, more than one form of
chorine species may diffuse through the membrane. In some
applications, the electrolyte behind the membrane may contain
chemicals to convert all the forms of chlorine to become one type
of oxide to be detected at the cathode. For example, if the
electrolyte contains iodide (F), chlorine may convert iodide to
iodine (I.sub.2) and the latter then is reduced at the cathode
quantitatively.
Examples 1-3
In Example 1 the electrode system of FIGS. 1-2 was left in
continuous contact with a zero-chlorine aqueous buffer solution of
pH 6.1 for 1 day, during which time a first voltage of -100 mV was
applied across the electrodes and the current was near zero. The
electrode system was then placed in contact with a
chlorine-containing aqueous buffered to a pH 6.1 and containing
approximately 1.64 ppm of free chlorine. The current in response to
the first voltage (sometimes referenced as a "data read voltage")
was measured and is shown as plot 100 in FIG. 3. Time=0 represents
the time of first exposure to chlorine-containing solution. The
plateau representing a current reading which is sufficiently stable
to provide an accurate calculation of free chlorine in a calibrated
system, was reached at a current reading of about 570 nA and was
achieved only after about 40 mins. The T90, represented by position
104, was not reached until about 20 minutes. This means that
anywhere from Time=0 to Time of about 20-40 minutes any current
reading taken on a calibrated system could not be reliably used to
calculate free chlorine. Note that in Example 1 the electrode was
then in contact with zero-chlorine solution for about 10 minutes at
position 108 (Time equals about 40 minutes) resulting in the zero
current reading of plot 100 beginning at about 40 minutes. However
this short period exposure to zero-chlorine solution did not have
profound effect on the electrode. The electrode responded to
chlorine relatively quickly with T90 being less than 4 mins. with
the testing setup.
In Example 2, the protocol of Example 1 was repeated except: the
electrode system was in continuous contact with the zero-chlorine
buffer solution for 2 days and then contacted with the
chlorine-containing solution containing 1.54 ppm of free chlorine;
and the electrode was in contact with zero-chlorine buffer solution
at Time equals about 30 minutes for a period of about 10 minutes.
The current measured in response to the read voltage is shown as
plot 120 in FIG. 3. It can be seen from plot 120 that the T90 was
about 20 minutes while the plateau was not reached until about 70
minutes after contact with the chlorine-containing solution.
In Example 3 the protocol of Example 1 was repeated except the
electrode system was in contact with the chlorine-free buffer
solution overnight during which time a program of pulses of the
second voltage were applied.
TABLE-US-00001 TABLE 1 Pulse voltage E2 (mV) (anode vs cathode)
-250 Pulse duration T2 (sec) 1 Pulse voltage E3 (mV) (anode vs
cathode) 760 Pulse duration T3 (sec) 1 Frequency (Hz) 0.5
After applying the pulse program as set out in Table 1, the
electrode system was then placed in contact with the
chlorine-containing solution and the first voltage (the data read
voltage) was again applied and the resulting current read. The
current measured in response is shown as plot 140 of FIG. 3. Note
that the electrode was dipped in zero-chlorine buffer solution in
this Example 3 twice (at Time equals about 30 mins and 60 mins). It
will be seen from plot 140 that after using pulse program of Table
1, the T90 was only about 5 minutes while the plateau was reached
in less than 10 minutes, much faster than in the case of Examples 1
or 2. Similarly, after exposing to zero-chlorine buffer solution
for short time period then returning to the chlorine solution at
about Time of 45 and 67 mins, the measured current rapidly reached
its maximum value, indicating that short term exposure to
zero-chlorine did not have any significant impact.
Examples 4, 5
Example 4 followed the same protocol as Example 1, except the
electrode system was exposed to the zero-chlorine buffer solution
for only 4 hours. At Time 0, the electrode system was back in
contact with a 1.15 ppm free chlorine solution at pH 6.1. The
resulting current is shown as plot 160 in FIG. 4. Note that the T90
was 8 minutes followed by a relatively long rise to an eventual
plateau value at about 20 minutes.
Example 5 followed the same protocol as Example 1 except that after
the electrode being running in zero-chlorine buffer under the first
voltage for 20 hours, a pulse of a second voltage of the same
program of Table 1 was applied for 5 minutes while the electrode
system was in contact with the chlorine-free buffer solution before
Time 0. At Time 0 the read voltage was applied while the electrode
system was in contact with a 1.15 ppm free chlorine solution at pH
6.1. The resulting current is shown as plot 180. Note that T90 of
plot 180 was only about 3.3 minutes.
Example 6
Example 6 followed the same protocol as Example 5 except the
electrode system was maintained in contact with the zero-chlorine
buffer solution over a weekend, and the pulses of the second
voltage were applied according to the protocol of Table 2 for 5
minutes before the electrode system was contacted with a chlorine
containing solution (0.89 ppm free chlorine) and the first voltage
was applied. Additionally, the electrode system was in contact with
zero-chlorine solution (at Times of about 14 and 29 minutes).
TABLE-US-00002 TABLE 2 Pulse voltage E2 (mV) (anode vs cathode)
-200 Pulse duration T2 (sec) 5 Pulse voltage E3 (mV) (anode vs
cathode) 780 Pulse duration T3 (sec) 5 Frequency (Hz) 0.1
The resulting current is shown as plot 200 of FIG. 5. Note again
the short T90 of about 4 minutes after the read voltage was
initially applied, and less than 2 minutes and about 3 minutes
after a short term contact with zero-chlorine buffer solution and
back to the chlorine solution at about Times of 18 minutes and 41
minutes.
Examples 7-8
Examples 7-8 followed the same protocol as Example 1 except as
noted in the following. The resulting current measurements are
shown as plots 220 and 240 in FIG. 6. In the case of plot 220, the
electrode system was left for 2 days in continuous contact with the
zero-chlorine buffer solution under the first voltage, the measured
current being zero. Second voltage pulses according to Table 3 were
then applied for 5 minutes prior to Time 0. At Time 0 the electrode
system was placed in contact with 1.31 ppm free chlorine in buffer
at pH 6.1. In the case of plot 240, the electrode system was left
for overnight in continuous contact with the zero-chlorine buffer
solution under the first voltage and then at Time 0 was in contact
with .about.1.1 ppm chlorine buffer without change in voltage. That
is, in plot 240 no pulses of the second voltage were applied. It
will be seen from a comparison of plots 220 and 240, that again the
use of the pulses of the second voltage resulted in a considerably
lower T90 than when no pulses were used, and plateau current levels
were achieved in a shorter Time.
TABLE-US-00003 TABLE 3 Pulse voltage E2 (mV) (anode vs cathode)
-200 Pulse duration T2 (sec) 1 Pulse voltage E3 (mV) (anode vs
cathode) 1100 Pulse duration T3 (sec) 1 Frequency (Hz) 0.5
Other pulse programs may be used than those described above. For
example, a single waveform as shown in FIG. 8 might be applied, or
the waveform of FIG. 8 could be repeated at a frequency of 0.5-20
Hz as shown in FIG. 9. Such a waveform of FIG. 9 may have the
parameters listed in Table 4 or Table 5 below and may be applied
during part or all of the time the electrode system is exposed to
the zero-chlorine or chlorine-containing aqueous solution. For
example, such a waveform may be continuously applied while the
electrode system is cycled through repeated periods of contact with
zero-chlorine and chlorine containing solutions. In FIGS. 8 and 9
current data may be taken at E1 (the "data read" or "first
voltage") during time T1-B after a delay of T1-A, e.g., 1000 ms, to
allow the system to stabilize. E2 and E3 represent the pulse
voltage ("second voltage").
TABLE-US-00004 TABLE 4 E1 (mV) (anode vs cathode) -100 T1 (ms) 1300
E2 (mV) (anode vs cathode) -200 T2 (ms) 100 E3 (mV) (anode vs
cathode) 0 T3 (ms) 100
TABLE-US-00005 TABLE 5 E1 (mV) (anode vs cathode) -100 T1 (ms) 1300
E2 (mV) (anode vs cathode) -600 T2 (ms) 80 E3 (mV) (anode vs
cathode) 1100 T3 (ms) 80
Other examples in which an apparatus and methods of any of the
above type can be used include those of below: a. Electrode system
is in continuous contact of chlorine solution. b. Electrode system
is in contact with low chlorine (e.g., <0.1 ppm) or
zero-chlorine solution for more than 4 hours. c. Electrode system
is in contact with contaminants that may deteriorate the activity
of the cathode.
Pulse programs may be flexible to include multiple voltages of
multiple time periods, as shown in FIGS. 8 and 9. For example, E2
and E3 may be in the range of -1200 mV to +1200 mV, pulse frequency
may be 0.1 to 1 Hz. Pulse programs may be applied for a fixed time
periodically, e.g., 5 minutes every 12 hours; or right before
measurement is started, e.g., 5 minutes before measurement is
started. Pulse programs may also be applied constantly when the
system is measuring chlorine, whether or not in chlorine containing
solution or in zero chlorine solution.
Particular embodiments of the present invention have been described
in detail above. For example, any of the embodiments might be used
for chlorine dioxide, bromine, and total oxidant detection.
However, it will be apparent that variations and modifications of
the described embodiments are possible. Accordingly, the present
invention is not limited by the embodiments described.
* * * * *